1. Field of the Invention
The present invention relates to a driving apparatus for a sewing machine, more particularly to the apparatus for driving an upper rotary shaft and a lower rotary shaft of the sewing machine.
2. Description of the Prior Art
In a conventional sewing machine for sewing a workpiece, e.g. a piece of cloth, or embroidering the cloth, a shuttle has to be rotated in the reverse direction to an upper rotary shaft. Therefore, according to an apparatus disclosed in Japanese Utility-model Publication for opposition No. 2-17576, each of an upper rotary shaft and a lower rotary shaft is provided with a bevel gear to rotate integrally therewith, respectively, and supported horizontally. The upper and lower rotary shafts are rotated together by a vertical shaft which is provided at its opposite ends with bevel gears to be meshed with the bevel gears of the upper and lower rotary shafts respectively, so that the upper rotary shaft is rotated by a driving source which is operatively connected to the upper rotary shaft thereby to rotate the lower rotary shaft in the reverse direction to the upper rotary shaft.
However, since the upper and lower rotary shafts are linked by the gears, a noise tends to be caused as they are rotated with the gears meshed with each other. Therefore, in general, lubricant is applied to the gears, and the working accuracy of the gears or a housing for receiving therein the gears and the assembling accuracy of them are required to be relatively high, which necessarily results in increase in cost.
Also, it has been proposed in Japanese Patent Laid-open Publication No. 54-135057 to employ a toothed belt for transmitting the rotational force of the driving source to the upper rotary shaft and transmitting the rotational force of the upper rotary shaft to the lower rotary shaft. As a result, the noise is reduced, while the lubricant is not necessarily applied, and the dimensional accuracy is not required to be so high in working or assembling operations, which result in decrease in cost.
According to the apparatus disclosed in the abovedescribed publication No. 54-135057, however, the upper and lower rotary shafts rotate in the same direction, so that a complex mechanism is required so as to rotate a bobbin in the reverse direction to the lower rotary shaft, which results in increase in cost. In addition, the driving source and the upper rotary shaft are driven by the toothed belt, respectively, so that a pair of toothed belts are necessitated, which results in increase in cost.